This invention relates to a thermal imaging medium for the recordation of information. More particularly, it relates to a laminar imaging medium having improved resistance to stress-induced delamination.
The provision of images by resort to media which rely upon the generation of heat patterns has been well known. Thermally imageable media are particularly advantageous inasmuch as they can be imaged without certain of the requirements attending the use of silver halide based media, such as darkroom processing and protection against ambient light. Moreover, the use of thermal imaging materials avoids the requirements of handling and disposing of silver-containing and other processing streams or effluent materials typically associated with the processing of silver halide based imaging materials.
Various methods and systems for preparing thermally generated symbols, patterns or other images have been reported. Examples of these can be found in U.S. Pat. No. 2,616,961 (issued Nov. 4, 1952 to J. Groak); in U.S. Pat. No. 3,257,942 (issued June 28, 1966 to W. Ritzerfeld, et al.); in U.S. Pat. No. 3,396,401 (issued Aug. 6, 1968 to K. K. Nonomura); in U.S. Pat. No. 3,592,644 (issued July 13, 1971 to M. N. Vrancken, et al.); in U.S. Pat. No. 3,632,376 (issued Jan. 4, 1972 to D. A. Newman); in U.S. Pat. No. 3,924,041 (issued Dec. 2, 1975 to M. Miyayama, et al.); in U.S. Pat. No. 4,123,578 (issued Oct. 31, 1978 to K. J. Perrington, et al.); in U.S. Pat. No. 4,157,412 (issued June 5, 1979 to K. S. Deneau); in Great Britain Patent Specification 1,156,996 (published July 2, 1969 by Pitney-Bowes, Inc.); and in International Patent Application No. PCT/US87/03249 of M. R. Etzel (published June 16, 1988, as International Publication No. WO 88/04237).
In the production of a thermally actuatable imaging material, it may be desirable and preferred that an image-forming substance be confined between a pair of sheets in the form of a laminate. Laminar thermal imaging materials are, for example, described in the aforementioned U.S. Pat. Nos. 3,924,041 and 4,157,412 and in the aforementioned International Patent Application No. PCT/US87/03249. It will be appreciated that the sheet elements of a laminar medium will afford protection of the image-forming substance confined therebetween against the effects of abrasion, rub-off and other physical stimuli. In addition, a laminar medium can be handled as a unitary structure, thus, obviating the requirement of bringing the respective sheets of a two-sheet imaging medium into proper position in the printer or other apparatus used for thermal imaging of the medium material.
In a laminar thermal imaging medium comprising at least a layer of image-forming substance confined between a pair of sheets, image formation may depend upon preferential adhesion of the image-forming substance to one of the sheets. Typically, such a laminar medium material will be designed such that the image-forming substance will be preferentially adherent to one of the sheets, before thermal actuation of regions of the laminar medium, and preferentially adherent to the other sheet in actuated or "exposed" regions. Separation of the sheets of the laminar medium material, in the case where there has been no thermal actuation or "exposure", provides a layer of image-forming substance on the one sheet to which it is preferentially adherent. Separation of the sheets of the medium material, in the case where the medium is exposed to radiation over its entire area and sufficient in intensity to reverse the preferential adhesion, provides the layer of image-forming substance on the opposite sheet. Accordingly, exposure of the medium selectively according to a predetermined pattern, and separation of the sheets after exposure, provides a pair of images on the respective sheets.
In the aforementioned International Application No. PCT/US87/03249, there is disclosed a thermal imaging medium which includes a layer of porous or particulate image-forming material and which is especially adapted to the provision of high resolution images by subjecting the medium to brief exposure to intense image-forming radiation. According to a preferred embodiment, the image-forming material (preferably, a layer of carbon black) is coated over the heat-activatable image-forming surface of a first sheet and is covered with a second laminated sheet, such that, the image-forming substance is confined between the sheets of a laminar thermal imaging medium. Upon exposure of the medium (for example, by laser scanning) and on separation of the sheets, a pair of images is obtained.
A first image comprises exposed portions of image-forming substance more firmly attached to the first sheet by heat activation of the heat-activatable image-forming surface thereof. A second image comprises non-exposed portions of the image-forming substance carried or transferred to the second sheet.
The respective images obtained by separating the sheets of an exposed thermal imaging medium having an image-forming substance confined therebetween may exhibit substantially different characteristics. Apart from the imagewise complementary nature of these images and the relation that each may bear as a "positive" or "negative" of an original, the respective images may differ in character. Differences may depend upon the properties of the image-forming substance, on the presence of additional layer(s) in the medium, and upon the manner in which such layers fail adhesively or cohesively upon separation of the sheets. Either of the pair of images may, for reasons of informational content, aesthetics or otherwise, be desirably considered the principal image. The principal image may, however, depending upon the aforementioned properties and modes of failure, exhibit decidedly inferior properties, such as poorer handling characteristics, durability and abrasion resistance, as compared with the complementary image of secondary importance.
In the production of thermal images from media of the type described in the aforementioned International Application, it will oftentimes be preferred, in the case of high density images, that the principal image be that which is formed by transfer of non-exposed regions of coated image-forming substance to a sheet separated from an imaged medium. It will be recognized that an alternative is to form a high density image on the opposed sheet by firmly attaching the image-forming substance in areas of exposure. This is the case because the medium provides complementary images and the desired high density image can be formed on either sheet by addressing the thermally actuatable medium according to which sheet shall bear the high density image. This alternative to the formation of a high density image is, however, disadvantageous since the areas of high density are created in areas of exposure (by activation of a heat-activatable image-forming zone or layer) and large areas of exposure require correspondingly large areas of laser actuation and energy utilization and highly accurate laser scanning and tracking. Errors in tracking will result in discontinuities (whiteness or voids) by failure to attach minute regions of image-forming substance and by their removal to the opposed sheet upon separation of the sheets. Owing to the psychophysical nature of human vision, minute regions of lightness (voids) against an expansive darkness tend to be noticeable.
It will, thus, be preferred that a high density image be the result of the transfer in non-exposed regions of coated and continuous regions of image-forming material (with minimal or no discontinuities or coverage voids), rather than the result of firm connection of high density regions of imaging material by laser-actuated operation of the heat-activatable image-forming surface, where tracking errors increase the possibility of creating noticeable areas of discontinuity (whiteness) against the expansive high density region.
Inasmuch as a laminar thermal imaging medium of the aforedescribed type will be designed such that the image-forming substance is preferentially adherent to only one of the sheets before and until thermal actuation, and will be designed to permit separation or peeling of the sheets after thermal exposure, the laminar medium material may exhibit an undesirable tendency to delaminate upon subjection to certain physical stresses that may be created during a manufacturing operation (e.g., bending, winding, cutting or stamping operations). It may be desirable in some instances to form a laminar medium from a pair of endless sheet or web materials and to then cut, slit or otherwise provide therefrom individual film units of predetermined size. A reciprocal cutting and stamping operation used for the cutting of individual film units may create stress influences in the medium, causing the sheets to separate at the interface of weakest adhesivity--typically, at the interface where, by thermal actuation, the preferential adhesion of the image-forming substance would be reversed.
In U.S. Pat. No. 5,200,297, issued Apr. 6, 1993 to Neal F. Kelly, there is disclosed and claimed a laminar thermal imaging medium including a polymeric stress-absorbing layer for reducing the tendency of such a medium material to delaminate as the result of application, during manufacture or use, of physical stresses to the medium material. As disclosed in such patent application, a polymeric stress-absorbing layer of compressible or elongatable material can be placed in close proximity to an interface having the greatest tendency to delaminate, so as to reduce the occurrence of undesired delamination during manufacture of the laminar medium or during use thereof in an imaging method or apparatus.
While the positioning of the polymeric stress-absorbing layer in a laminar thermal imaging medium can vary, consistent with the desired objective of minimizing undesired delamination, the required properties of the layer may adversely affect other desired properties of either the thermal imaging medium or an image obtained therefrom. For example, in the manufacture of a laminar imaging medium of the type described in the aforementioned International Application No. PCT/US87/03249, a soft adhesive material can be employed as an adhesive layer for the lamination of a second sheet to a first sheet carrying the layer of image-forming substance. Upon exposure and separation of the first and second sheets, first and second images, as aforedescribed, are provided. The second image comprises non-exposed portions of the image-forming substance carried or transferred to the second sheet with the aid of the adhesive thereof. The adhesive material is effective for the carrying or removal of unexposed image-forming substance and for the provision of stress-absorbing properties which minimize undesired delamination. The adhesive also serves, however, as a base for the second image which is formed in image-forming substance. Softness of the adhesive base tends to reduce the durability of the image which, for reasons mentioned hereinbefore, may be the principal image.
It will be appreciated that a thermal imaging medium which is designed for separation of a pair of sheets and images will be especially desirable where the imaging medium is resistant to undesired delamination during the manufacture thereof and is adapted to provide an image having satisfactory handling and durability characteristics.